Many electrical devices, especially computers and related network equipment, use power strips for electric power distribution. In such power strips, if the power consumed exceeds the limit of the circuit breaker, the electrical device will be abruptly disabled to prevent damage to the device. However, in many applications, such as web or e-mail servers, sustained operation is expected. Consequently, it is often desirable to measure the current and/or the voltage consumed by a power strip.
Determining the amount of current being consumed by an individual power strip is not a simple matter. While current can be measured at a power distribution box (e.g., wall-mounted circuit breaker) due to the exposure of individual conductors, the power measured is usually for multiple power strips on a given power circuit and not individual devices. Such a value is not particularly useful when conducting power strip-specific analysis (e.g., determining how many more devices can be plugged into a given power strip before the individual power strip circuit breaker trips).
Current measurements in conductors are commonly accomplished using a current transformer. A current transformer is designed to produce either an alternating current or alternating voltage proportional to the current being measured due to the electromagnetic field generated by the measured current. Referring to FIG. 1, a current transformer 100 common to the art is presented. Current transformers are often constructed by passing a single primary conductor (the primary turn) through a well-insulated toroidal core 101 wrapped with multiple turns of a transformer conductor 102 (the secondary turns). The toroidal core 101 may be composed of ferrous materials such as iron, silicon steel, carbonyl iron or other such materials common in the art. However, such toroidal transformers 100 are not effective for use in pre-existing electrical cables of power strips because the incoming 103 and outgoing 104 electrical currents create opposing magnetic fields which cancel each other.
As such, in order to determine the amount of current in a given conductor of a power strip cable 200 using a wire-wrapped toroidal current transformer 201, a portion of the insulating cable cover 202 must be cut away so a single conductor 203 can be withdrawn from the insulating cover 202 as shown in FIG. 2. The exposed conductor 203 can be cut 204 and a current transformer 201 inserted. For temporary measurements, an electrician's portable ammeter may be used (not shown). Several disadvantages exist with respect to such a method. First, one of the conductors 203 must be exposed. Second, the devices being powered by the power cable 200 should be powered down during addition of the current transformer 201 as a safety precaution, thereby causing undesired system downtime for the devices connected to the power strip.
The most common method used to measure current in an electrical cable requires an electrician to power-off the devices connected to the power strip and temporarily insert a break-out box between the power source (wall or floor receptacle) and the power strip itself. This box exposes the individual conductors so that a clamp-on ammeter can be used to manually measure the current. This method has three drawbacks. First, the equipment has to be powered off before the break-out box can be inserted, again resulting undesirable system downtime, or, alternately, removing the outer insulation on the conductors while the circuit is powered to avoid system downtime, thereby creating a potentially dangerous situation. Second, the value obtained by such manual measurements may quickly become out-of-date as equipment is added or removed from the power strip, thereby requiring repeated measurements.
Consequently, it would be advantageous to provide a current and/or voltage measurement device that allows a user to easily and safely determine current flow and/or voltage in a power cable without modifying a cable by utilizing a semi-permanent wrap-around current and/or voltage monitor.